Differential Assembly of Inwardly Rectifying K+ Channel Subunits, Kir4.1 and Kir5.1, in Brain Astrocytes

Astrocyte Kir4.1 expression level territorially controls excitatory transmission in the brain

Intense brain activity elevates extracellular potassium, potentially leading to overexcitation and seizures. Astrocytes are crucial for restoring healthy potassium levels, and an emerging focus on their Kir4.1 channels has reopened the quest into the underlying mechanisms. We find that the Kir4.1 level in individual astrocytes sets the kinetics of their potassium and glutamate uptake current. Combining electrophysiology with multiplexed optical sensor imaging and FLIM reveals that rises in extracellular potassium would normally boost presynaptic Ca2+ entry and release probability at excitatory synapses unless such synapses are surrounded by the Kir4.1-overexpressing astrocytes. Inside the territories of Kir4.1-overexpressing astrocytes, high-frequency afferent stimulation fails to induce long-term synaptic potentiation, and the high-potassium waves of cortical spreading depolarization are markedly attenuated. Biophysical exploration explains how astrocytes can regulate local potassium homeostasis by engaging Kir4.1 channels. Our findings thus point to a fundamental astrocytic mechanism that can restrain the activity-driven rise of excitability in brain circuits.

Kir5.1 regulates Kir4.2 expression and is a key component of the 50-pS inwardly rectifying potassium channel in basolateral membrane of mouse proximal tubules

Kir5.1 encoded by Kcnj16 is an inwardly rectifying K+ channel subunit, and it possibly interacts with Kir4.2 subunit encoded by Kcnj15 for assembling a Kir4.2/Kir5.1 heterotetramer in the basolateral membrane of mouse proximal tubule. We now used patch clamp technique to examine basolateral K+ channels of mouse proximal tubule (PT) and an immunoblotting/immunofluorescence (IF) staining microscope to examine Kir4.2 expression in wild-type and Kir5.1-knockout mice. IF staining shows that Kir4.2 was exclusively expressed in the proximal tubule, whereas Kir5.1 was expressed in the proximal tubule and distal nephrons including distal convoluted tubule. Immunoblotting showed that the expression of Kir4.2 monomer was lower in Kir5.1-knockout mice than that in the wild-type mice. In contrast, Kir4.1 monomer expression was increased in Kir5.1 knockout mice. IF images further demonstrated that the basolateral membrane staining of Kir4.2 was significantly decreased in Kir5.1 knockout mice. This is in sharp contrast to Kir4.1, which also interacts with Kir5.1 in the distal nephron, and IF images show that Kir4.1 membrane expression was still visible and unchanged in Kir5.1 knockout mice. The single channel recording detected a 50-pS inwardly rectifying K+ channel, presumably a Kir4.2/Kir5.1 heterotetramer, in the basolateral membrane of the proximal tubule of Kir5.1 wild-type mice. However, this 50-pS K+ channel was completely absent in the basolateral membrane of the proximal tubule of Kir5.1 knockout mice. Moreover, the membrane potential of the proximal tubule was less negative in Kir5.1 knockout mice than wild-type mice. We conclude that Kir5.1 is essential for assembling basolateral 50-pS K+ channel in proximal tubule and that deletion of Kir5.1 decreased Kir4.2 expression in the proximal tubule thereby decreasing the basolateral K+ conductance and the membrane potentials.NEW & NOTEWORTHY Our study provides direct evidence for the notion that Kir5.1 is a key component of a 50-60 pS inwardly-rectifying-K+ channel, a main type K+ channel in the basolateral-membrane of PT. Also, we demonstrate that deletion of Kir5.1 decreased Kir4.2 protein expression including the basolateral-membrane in PT. Finally, depolarization of PT-membrane- potential in Kir5.1-knockout mice suggests that Kir4.2 alone is not able to sustain basolateral K+ conductance of the PT in the absence of Kir5.1.

Physiology of neuroglia of the central nervous system

Neuroglia of the central nervous system (CNS) are a diverse and highly heterogeneous population of cells of ectodermal, neuroepithelial origin (macroglia, that includes astroglia and oligodendroglia) and mesodermal, myeloid origin (microglia). Neuroglia are primary homeostatic cells of the CNS, responsible for the support, defense, and protection of the nervous tissue. The extended class of astroglia (which includes numerous parenchymal astrocytes, such as protoplasmic, fibrous, velate, marginal, etc., radial astrocytes such as Bergmann glia, Muller glia, etc., and ependymoglia lining the walls of brain ventricles and central canal of the spinal cord) is primarily responsible for overall homeostasis of the nervous tissue. Astroglial cells control homeostasis of ions, neurotransmitters, hormones, metabolites, and are responsible for neuroprotection and defense of the CNS. Oligodendroglia provide for myelination of axons, hence supporting and sustaining CNS connectome. Microglia are tissue macrophages adapted to the CNS environment which contribute to the host of physiologic functions including regulation of synaptic connectivity through synaptic pruning, regulation of neurogenesis, and even modifying neuronal excitability. Neuroglial cells express numerous receptors, transporters, and channels that allow neuroglia to perceive and follow neuronal activity. Activation of these receptors triggers intracellular ionic signals that govern various homeostatic cascades underlying glial supportive and defensive capabilities. Ionic signaling therefore represents the substrate of glial excitability.

Glial cells in the mammalian olfactory bulb

The mammalian olfactory bulb (OB), an essential part of the olfactory system, plays a critical role in odor detection and neural processing. Historically, research has predominantly focused on the neuronal components of the OB, often overlooking the vital contributions of glial cells. Recent advancements, however, underscore the significant roles that glial cells play within this intricate neural structure. This review discus the diverse functions and dynamics of glial cells in the mammalian OB, mainly focused on astrocytes, microglia, oligodendrocytes, olfactory ensheathing cells, and radial glia cells. Each type of glial contributes uniquely to the OB's functionality, influencing everything from synaptic modulation and neuronal survival to immune defense and axonal guidance. The review features their roles in maintaining neural health, their involvement in neurodegenerative diseases, and their potential in therapeutic applications for neuroregeneration. By providing a comprehensive overview of glial cell types, their mechanisms, and interactions within the OB, this article aims to enhance our understanding of the olfactory system's complexity and the pivotal roles glial cells play in both health and disease.

Astrocyte-like subpopulation of NG2 glia in the adult mouse cortex exhibits characteristics of neural progenitor cells

Glial cells expressing neuron-glial antigen 2 (NG2), also known as oligodendrocyte progenitor cells (OPCs), play a critical role in maintaining brain health. However, their ability to differentiate after ischemic injury is poorly understood. The aim of this study was to investigate the properties and functions of NG2 glia in the ischemic brain. Using transgenic mice, we selectively labeled NG2-expressing cells and their progeny in both healthy brain and after focal cerebral ischemia (FCI). Using single-cell RNA sequencing, we classified the labeled glial cells into five distinct subpopulations based on their gene expression patterns. Additionally, we examined the membrane properties of these cells using the patch-clamp technique. Of the identified subpopulations, three were identified as OPCs, whereas the fourth subpopulation had characteristics indicative of cells likely to develop into oligodendrocytes. The fifth subpopulation of NG2 glia showed astrocytic markers and had similarities to neural progenitor cells. Interestingly, this subpopulation was present in both healthy and post-ischemic tissue; however, its gene expression profile changed after ischemia, with increased numbers of genes related to neurogenesis. Immunohistochemical analysis confirmed the temporal expression of neurogenic genes and showed an increased presence of NG2 cells positive for Purkinje cell protein-4 at the periphery of the ischemic lesion 12 days after FCI, as well as NeuN-positive NG2 cells 28 and 60 days after injury. These results suggest the potential development of neuron-like cells arising from NG2 glia in the ischemic tissue. Our study provides insights into the plasticity of NG2 glia and their capacity for neurogenesis after stroke.

Ion Channels and Ionotropic Receptors in Astrocytes: Physiological Functions and Alterations in Alzheimer's Disease and Glioblastoma

The human brain is composed of nearly one hundred billion neurons and an equal number of glial cells, including macroglia, i.e., astrocytes and oligodendrocytes, and microglia, the resident immune cells of the brain. In the last few decades, compelling evidence has revealed that glial cells are far more active and complex than previously thought. In particular, astrocytes, the most abundant glial cell population, not only take part in brain development, metabolism, and defense against pathogens and insults, but they also affect sensory, motor, and cognitive functions by constantly modulating synaptic activity. Not surprisingly, astrocytes are actively involved in neurodegenerative diseases (NDs) and other neurological disorders like brain tumors, in which they rapidly become reactive and mediate neuroinflammation. Reactive astrocytes acquire or lose specific functions that differently modulate disease progression and symptoms, including cognitive impairments. Astrocytes express several types of ion channels, including K+, Na+, and Ca2+ channels, transient receptor potential channels (TRP), aquaporins, mechanoreceptors, and anion channels, whose properties and functions are only partially understood, particularly in small processes that contact synapses. In addition, astrocytes express ionotropic receptors for several neurotransmitters. Here, we provide an extensive and up-to-date review of the roles of ion channels and ionotropic receptors in astrocyte physiology and pathology. As examples of two different brain pathologies, we focus on Alzheimer's disease (AD), one of the most diffuse neurodegenerative disorders, and glioblastoma (GBM), the most common brain tumor. Understanding how ion channels and ionotropic receptors in astrocytes participate in NDs and tumors is necessary for developing new therapeutic tools for these increasingly common neurological conditions.

Non-trivial dynamics in a model of glial membrane voltage driven by open potassium pores

Despite the molecular evidence that a nearly linear steady-state current-voltage relationship in mammalian astrocytes reflects a total current resulting from more than one differentially regulated K+ conductance, detailed ordinary differential equation (ODE) models of membrane voltage Vm are still lacking. Various experimental results reporting altered rectification of the major Kir currents in glia, dominated by Kir4.1, have motivated us to develop a detailed model of Vm dynamics incorporating the weaker potassium K2P-TREK1 current in addition to Kir4.1, and study the stability of the resting state Vr. The main question is whether, with the loss of monotonicity in glial I-V curve resulting from altered Kir rectification, the nominal resting state Vr remains stable, and the cell retains the trivial, potassium electrode behavior with Vm after EK. The minimal two-dimensional model of Vm near Vr showed that an N-shape deformed Kir I-V curve induces multistability of Vm in a model that incorporates K2P activation kinetics, and nonspecific K+ leak currents. More specifically, an asymmetrical, nonlinear decrease of outward Kir4.1 conductance, turning the channels into inward rectifiers, introduces instability of Vr. That happens through a robust bifurcation giving birth to a second, more depolarized stable resting state Vdr > -10 mV. Realistic recordings from electrographic seizures were used to perturb the model. Simulations of the model perturbed by constant current through gap junctions and seizure-like discharges as local field potentials led to depolarization and switching of Vm between the two stable states, in a downstate-upstate manner. In the event of prolonged depolarizations near Vdr, such catastrophic instability would affect all aspects of the glial function, from metabolic support to membrane transport, and practically all neuromodulatory roles assigned to glia.

Kir4.1 is specifically expressed and active in non-myelinating Schwann cells

Non-myelinating Schwann cells (NMSC) play important roles in peripheral nervous system formation and function. However, the molecular identity of these cells remains poorly defined. We provide evidence that Kir4.1, an inward-rectifying K+ channel encoded by the KCNJ10 gene, is specifically expressed and active in NMSC. Immunostaining revealed that Kir4.1 is present in terminal/perisynaptic SCs (TPSC), synaptic glia at neuromuscular junctions (NMJ), but not in myelinating SCs (MSC) of adult mice. To further examine the expression pattern of Kir4.1, we generated BAC transgenic Kir4.1-CreERT2 mice and crossed them to the tdTomato reporter line. Activation of CreERT2 with tamoxifen after the completion of myelination onset led to robust expression of tdTomato in NMSC, including Remak Schwann cells (RSC) along peripheral nerves and TPSC, but not in MSC. In contrast, activating CreERT2 before and during the onset of myelination led to tdTomato expression in NMSC and MSC. These observations suggest that immature SC express Kir4.1, and its expression is then downregulated selectively in myelin-forming SC. In support, we found that while activating CreERT2 induces tdTomato expression in immature SC, it fails to induce tdTomato in MSC associated with sensory axons in culture. NMSC derived from neonatal sciatic nerve were shown to express Kir4.1 and exhibit barium-sensitive inwardly rectifying macroscopic K+ currents. Thus, this study identified Kir4.1 as a potential modulator of immature SC and NMSC function. Additionally, it established a novel transgenic mouse line to introduce or delete genes in NMSC.

Diverse functions of the inward-rectifying potassium channel Kir5.1 and its relationship with human diseases

The inward-rectifying potassium channel subunit Kir5.1, encoded by Kcnj16, can form functional heteromeric channels (Kir4.1/5.1 and Kir4.2/5.1) with Kir4.1 (encoded by Kcnj10) or Kir4.2 (encoded by Kcnj15). It is expressed in the kidneys, pancreas, thyroid, brain, and other organs. Although Kir5.1 cannot form functional homomeric channels in most cases, an increasing number of studies in recent years have found that the functions of this subunit should not be underestimated. Kir5.1 can confer intracellular pH sensitivity to Kir4.1/5.1 channels, which can act as extracellular potassium sensors in the renal distal convoluted tubule segment. This segment plays an important role in maintaining potassium and acid-base balances. This review summarizes the various pathophysiological processes involved in Kir5.1 and the expression changes of Kir5.1 as a differentially expressed gene in various cancers, as well as describing several other disease phenotypes caused by Kir5.1 dysfunction.

Regulation of Potassium and Chloride Concentrations in Nervous Tissue as a Method of Anticonvulsant Therapy

Abstract

Under some pathological conditions, such as pharmacoresistant epilepsy, status epilepticus or certain forms of genetic abnormalities, spiking activity of GABAergic interneurons may enhance excitation processes in neuronal circuits and provoke the generation of ictal discharges. As a result, anticonvulsants acting on the GABAergic system may be ineffective or even increase seizure activity. This paradoxical effect of the inhibitory system is due to ionic imbalances in nervous tissue. This review addresses the mechanisms of ictal discharge initiation in neuronal networks due to the imbalance of chloride and potassium ions, as well as possible ways to regulate ionic concentrations. Both the enhancement (or attenuation) of the activity of certain neuronal ion transporters and ion pumps and their additional expression via gene therapy can be effective in suppressing seizure activity caused by ionic imbalances. The Na+–K+-pump, NKCC1 and KCC2 cotransporters are important for maintaining proper K+ and Cl– concentrations in nervous tissue, having been repeatedly considered as pharmacological targets for antiepileptic exposures. Further progress in this direction is hampered by the lack of sufficiently selective pharmacological tools and methods for providing effective drug delivery to the epileptic focus. The use of the gene therapy techniques, such as overexpressing of the KCC2 transporter in the epileptic focus, seems to be a more promising approach. Another possible direction could be the use of optogenetic tools, namely specially designed light-activated ion pumps or ion channels. In this case, photon energy can be used to create the required gradients of chloride and potassium ions, although these methods also have significant limitations which complicate their rapid introduction into medicine.

Glia as a key factor in cell volume regulation processes of the central nervous system

Brain edema is a pathological condition with potentially fatal consequences, related to cerebral injuries such as ischemia, chronic renal failure, uremia, and diabetes, among others. Under these pathological states, the cell volume control processes are fully compromised, because brain cells are unable to regulate the movement of water, mainly regulated by osmotic gradients. The processes involved in cell volume regulation are homeostatic mechanisms that depend on the mobilization of osmolytes (ions, organic molecules, and polyols) in the necessary direction to counteract changes in osmolyte concentration in response to water movement. The expression and coordinated function of proteins related to the cell volume regulation process, such as water channels, ion channels, and other cotransport systems in the glial cells, and considering the glial cell proportion compared to neuronal cells, leads to consider the astroglial network the main regulatory unit for water homeostasis in the central nervous system (CNS). In the last decade, several studies highlighted the pivotal role of glia in the cell volume regulation process and water homeostasis in the brain, including the retina; any malfunction of this astroglial network generates a lack of the ability to regulate the osmotic changes and water movements and consequently exacerbates the pathological condition.

Astrocytes profiling in acute hepatic encephalopathy: Possible enrolling of glial fibrillary acidic protein, tumor necrosis factor-alpha, inwardly rectifying potassium channel (Kir 4.1) and aquaporin-4 in rat cerebral cortex

Hepatic encephalopathy (HE) is a neurological disarray manifested as a sequel to chronic and acute liver failure (ALF). A potentially fatal consequence of ALF is brain edema with concomitant astrocyte enlargement. This study aims to outline the role of astrocytes in acute HE and shed light on the most critical mechanisms driving this role. Rats were allocated into two groups. Group 1, the control group, received the vehicle. Group 2, the TAA group, received TAA (300 mg/kg) for 3 days. Serum AST, ALT, and ammonia were determined. Liver and cerebral cortical sections were processed for hematoxylin and eosin staining. Additionally, mRNA expression and immunohistochemical staining of cortical GFAP, TNFα, Kir4.1, and AQP4 were performed. Cortical sections from the TAA group demonstrated neuropil vacuolation and astrocytes enlargement with focal gliosis. GFAP, TNFα, and AQP4 revealed increased mRNA expression, positive immunoreactivity, and a positive correlation to brain water content. In contrast, Kir 4.1 showed decreased mRNA expression and immunoreactivity and a negative correlation to brain water content. In conclusion, our findings revealed altered levels of TNFα, Kir 4.1, GFAP, and AQP4 in HE-associated brain edema. A more significant dysregulation of Kir 4.1 and TNFα was observed compared to AQP4 and GFAP.

EAST/SeSAME Syndrome and Beyond: The Spectrum of Kir4.1- and Kir5.1-Associated Channelopathies

In 2009, two groups independently linked human mutations in the inwardly rectifying K+ channel Kir4.1 (gene name KCNJ10) to a syndrome affecting the central nervous system (CNS), hearing, and renal tubular salt reabsorption. The autosomal recessive syndrome has been named EAST (epilepsy, ataxia, sensorineural deafness, and renal tubulopathy) or SeSAME syndrome (seizures, sensorineural deafness, ataxia, intellectual disability, and electrolyte imbalance), accordingly. Renal dysfunction in EAST/SeSAME patients results in loss of Na+, K+, and Mg2+ with urine, activation of the renin-angiotensin-aldosterone system, and hypokalemic metabolic alkalosis. Kir4.1 is highly expressed in affected organs: the CNS, inner ear, and kidney. In the kidney, it mostly forms heteromeric channels with Kir5.1 (KCNJ16). Biallelic loss-of-function mutations of Kir5.1 can also have disease significance, but the clinical symptoms differ substantially from those of EAST/SeSAME syndrome: although sensorineural hearing loss and hypokalemia are replicated, there is no alkalosis, but rather acidosis of variable severity; in contrast to EAST/SeSAME syndrome, the CNS is unaffected. This review provides a framework for understanding some of these differences and will guide the reader through the growing literature on Kir4.1 and Kir5.1, discussing the complex disease mechanisms and the variable expression of disease symptoms from a molecular and systems physiology perspective. Knowledge of the pathophysiology of these diseases and their multifaceted clinical spectrum is an important prerequisite for making the correct diagnosis and forms the basis for personalized therapies.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Dynamic expression of homeostatic ion channels in differentiated cortical astrocytes in vitro

The capacity of astrocytes to adapt their biochemical and functional features upon physiological and pathological stimuli is a fundamental property at the basis of their ability to regulate the homeostasis of the central nervous system (CNS). It is well known that in primary cultured astrocytes, the expression of plasma membrane ion channels and transporters involved in homeostatic tasks does not closely reflect the pattern observed in vivo. The individuation of culture conditions that promote the expression of the ion channel array found in vivo is crucial when aiming at investigating the mechanisms underlying their dynamics upon various physiological and pathological stimuli. A chemically defined medium containing growth factors and hormones (G5) was previously shown to induce the growth, differentiation, and maturation of primary cultured astrocytes. Here we report that under these culture conditions, rat cortical astrocytes undergo robust morphological changes acquiring a multi-branched phenotype, which develops gradually during the 2-week period of culturing. The shape changes were paralleled by variations in passive membrane properties and background conductance owing to the differential temporal development of inwardly rectifying chloride (Cl⁻) and potassium (K⁺) currents. Confocal and immunoblot analyses showed that morphologically differentiated astrocytes displayed a large increase in the expression of the inward rectifier Cl⁻ and K⁺ channels ClC-2 and Kir4.1, respectively, which are relevant ion channels in vivo. Finally, they exhibited a large diminution of the intermediate filaments glial fibrillary acidic protein (GFAP) and vimentin which are upregulated in reactive astrocytes in vivo. Taken together the data indicate that long-term culturing of cortical astrocytes in this chemical-defined medium promotes a quiescent functional phenotype. This culture model could aid to address the regulation of ion channel expression involved in CNS homeostasis in response to physiological and pathological challenges.

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

A developmental stage- and Kidins220-dependent switch in astrocyte responsiveness to brain-derived neurotrophic factor

Astroglial cells are key to maintain nervous system homeostasis. Neurotrophins are known for their pleiotropic effects on neuronal physiology but also exert complex functions to glial cells. Here, we investigated (i) the signaling competence of mouse embryonic and postnatal primary cortical astrocytes exposed to brain-derived neurotrophic factor (BDNF) and, (ii) the role of kinase D-interacting substrate of 220 kDa (Kidins220), a transmembrane scaffold protein that mediates neurotrophin signaling in neurons. We found a shift from a kinase-based response in embryonic cells to a response predominantly relying on intracellular Ca2+ transients [Ca2+]i within postnatal cultures, associated with a decrease in the synthesis of full-length BDNF receptor TrkB, with Kidins220 contributing to the BDNF-activated kinase and [Ca2+]i pathways. Finally, Kidins220 participates in the homeostatic function of astrocytes by controlling the expression of the ATP-sensitive inward rectifier potassium channel 10 (Kir4.1) and the metabolic balance of embryonic astrocytes. Overall, our data contribute to the understanding of the complex role played by astrocytes within the central nervous system, and identify Kidins220 as a novel actor in the increasing number of pathologies characterized by astrocytic dysfunctions. This article has an associated First Person interview with the first authors of the paper.

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Defects in KCNJ16 Cause a Novel Tubulopathy with Hypokalemia, Salt Wasting, Disturbed Acid-Base Homeostasis, and Sensorineural Deafness

BACKGROUND

The transepithelial transport of electrolytes, solutes, and water in the kidney is a well-orchestrated process involving numerous membrane transport systems. Basolateral potassium channels in tubular cells not only mediate potassium recycling for proper Na⁺,K⁺-ATPase function but are also involved in potassium and pH sensing. Genetic defects in KCNJ10 cause EAST/SeSAME syndrome, characterized by renal salt wasting with hypokalemic alkalosis associated with epilepsy, ataxia, and sensorineural deafness.

METHODS

A candidate gene approach and whole-exome sequencing determined the underlying genetic defect in eight patients with a novel disease phenotype comprising a hypokalemic tubulopathy with renal salt wasting, disturbed acid-base homeostasis, and sensorineural deafness. Electrophysiologic studies and surface expression experiments investigated the functional consequences of newly identified gene variants.

RESULTS

We identified mutations in the KCNJ16 gene encoding KCNJ16, which along with KCNJ15 and KCNJ10, constitutes the major basolateral potassium channel of the proximal and distal tubules, respectively. Coexpression of mutant KCNJ16 together with KCNJ15 or KCNJ10 in Xenopus oocytes significantly reduced currents.

CONCLUSIONS

Biallelic variants in KCNJ16 were identified in patients with a novel disease phenotype comprising a variable proximal and distal tubulopathy associated with deafness. Variants affect the function of heteromeric potassium channels, disturbing proximal tubular bicarbonate handling as well as distal tubular salt reabsorption.

Ion Channels and Electrophysiological Properties of Astrocytes: Implications for Emergent Stimulation Technologies

Astrocytes comprise a heterogeneous cell population characterized by distinct morphologies, protein expression and function. Unlike neurons, astrocytes do not generate action potentials, however, they are electrically dynamic cells with extensive electrophysiological heterogeneity and diversity. Astrocytes are hyperpolarized cells with low membrane resistance. They are heavily involved in the modulation of K⁺ and express an array of different voltage-dependent and voltage-independent channels to help with this ion regulation. In addition to these K⁺ channels, astrocytes also express several different types of Na⁺ channels; intracellular Na⁺ signaling in astrocytes has been linked to some of their functional properties. The physiological hallmark of astrocytes is their extensive intracellular Ca²⁺ signaling cascades, which vary at the regional, subregional, and cellular levels. In this review article, we highlight the physiological properties of astrocytes and the implications for their function and influence of network and synaptic activity. Furthermore, we discuss the implications of these differences in the context of optogenetic and DREADD experiments and consider whether these tools represent physiologically relevant techniques for the interrogation of astrocyte function.

Electrophysiological features of sleep in children with Kir4.1 channel mutations and Autism-Epilepsy phenotype: a preliminary study

Study Objectives

Recently, a role for gain-of-function (GoF) mutations of the astrocytic potassium channel Kir4.1 (KCNJ10 gene) has been proposed in subjects with Autism–Epilepsy phenotype (AEP). Epilepsy and autism spectrum disorder (ASD) are common and complexly related to sleep disorders. We tested whether well characterized mutations in KCNJ10 could result in specific sleep electrophysiological features, paving the way to the discovery of a potentially relevant biomarker for Kir4.1-related disorders.

Methods

For this case–control study, we recruited seven children with ASD either comorbid or not with epilepsy and/or EEG paroxysmal abnormalities (AEP) carrying GoF mutations of KCNJ10 and seven children with similar phenotypes but wild-type for the same gene, comparing period-amplitude features of slow waves detected by fronto-central bipolar EEG derivations (F3-C3, F4-C4, and Fz-Cz) during daytime naps.

Results

Children with Kir4.1 mutations displayed longer slow waves periods than controls, in Fz-Cz (mean period = 112,617 ms ± SE = 0.465 in mutated versus mean period = 105,249 ms ± SE = 0.375 in controls, p < 0.001). An analog result was found in F3-C3 (mean period = 125,706 ms ± SE = 0.397 in mutated versus mean period = 120,872 ms ± SE = 0.472 in controls, p < 0.001) and F4-C4 (mean period = 127,914 ms ± SE = 0.557 in mutated versus mean period = 118,174 ms ± SE = 0.442 in controls, p < 0.001).

Conclusion

This preliminary finding suggests that period-amplitude slow wave features are modified in subjects carrying Kir4.1 GoF mutations. Potential clinical applications of this finding are discussed.

Nedd4-2 haploinsufficiency in mice causes increased seizure susceptibility and impaired Kir4.1 ubiquitination

Neural precursor cell expressed developmentally down-regulated gene 4-like (NEDD4-2) encodes a ubiquitin E3 ligase that is involved in epileptogenesis with mechanisms needing further investigation. We constructed a novel Nedd4-2^+/- mouse model with half level of both Nedd4-2 long and short isoforms in the brain. Nedd4-2 haploinsufficiency caused increased susceptibility and severity of pentylenetetrazole (PTZ)-induced seizures. Of the 3379 proteins identified by the hippocampal proteomic analysis, 55 were considered altered in Nedd4-2^+/- mice compared with wild-type control, among which the inwardly rectifying K⁺ channel Kir4.1 was up-regulated by 1.83-fold. Kir4.1 was subsequently confirmed to be less ubiquitinated in response to comprised Nedd4-2 in mouse brains and C6 cells. Kir4.1 associated with Nedd4-2 through the threonine³¹²-proline motif in the intracellular domain by target mutagenesis. Adaptor protein 14-3-3 facilitated Nedd4-2-mediated ubiquitination of Kir4.1. Our data consolidate the detailed molecular mechanism of Nedd4-2-mediated Kir4.1 ubiquitination, and provide a possible relationship between increased seizure susceptibility and impaired Kir4.1 ubiquitination in the brain.

Kir 5.1-dependent CO2 /H+ -sensitive currents contribute to astrocyte heterogeneity across brain regions

Astrocyte heterogeneity is an emerging concept in which astrocytes within or between brain regions show variable morphological and/or gene expression profiles that presumably reflect different functional roles. Recent evidence indicates that retrotrapezoid nucleus (RTN) astrocytes sense changes in tissue CO2/ H+ to regulate respiratory activity; however, mechanism(s) by which they do so remain unclear. Alterations in inward K+ currents represent a potential mechanism by which CO2 /H+ signals may be conveyed to neurons. Here, we use slice electrophysiology in rats of either sex to show that RTN astrocytes intrinsically respond to CO2 /H+ by inhibition of an inward rectifying potassium (Kir ) conductance and depolarization of the membrane, while cortical astrocytes do not exhibit such CO2 /H+ -sensitive properties. Application of Ba2+ mimics the effect of CO2 /H+ on RTN astrocytes as measured by reductions in astrocyte Kir -like currents and increased RTN neuronal firing. These CO2 /H+ -sensitive currents increase developmentally, in parallel to an increased expression in Kir 4.1 and Kir 5.1 in the brainstem. Finally, the involvement of Kir 5.1 in the CO2 /H+ -sensitive current was verified using a Kir5.1 KO rat. These data suggest that Kir inhibition by CO2 /H+ may govern the degree to which astrocytes mediate downstream chemoreceptive signaling events through cell-autonomous mechanisms. These results identify Kir channels as potentially important regional CO2 /H+ sensors early in development, thus expanding our understanding of how astrocyte heterogeneity may uniquely support specific neural circuits and behaviors.

Correlation between Kir4.1 expression and barium-sensitive currents in rat and human glioma cell lines

Gliomas are the most common primary brain tumors and often become apparent through symptomatic epileptic seizures. Glial cells express the inwardly rectifying K+ channel Kir4.1 playing a major role in K+ buffering, and are presumably involved in facilitating epileptic hyperexcitability. We therefore aimed to investigate the molecular and functional expression of Kir4.1 channels in cultured rat and human glioma cells. Quantitative PCR showed reduced expression of Kir4.1 in rat C6 and F98 cells as compared to control. In human U-87MG cells and in patient-derived low-passage glioblastoma cultures, Kir4.1 expression was also reduced as compared to autopsy controls. Testing Kir4.1 function using whole-cell patch-clamp experiments on rat C6 and two human low-passage glioblastoma cell lines (HROG38 and HROG05), we found a significantly depolarized resting membrane potential (RMP) in HROG05 (-29 ± 2 mV, n = 11) compared to C6 (-71 ± 1 mV, n = 12, P < 0.05) and HROG38 (-60 ± 2 mV, n = 12, P < 0.05). Sustained K+ inward or outward currents were sensitive to Ba2+ added to the bath solution in HROG38 and C6 cells, but not in HROG05 cells, consistent with RMP depolarization. While immunocytochemistry confirmed Kir4.1 in all three cell lines including HROG05, we found that aquaporin-4 and Kir5.1 were also significantly reduced suggesting that the Ba2+-sensitive K+ current is generally impaired in glioma tissue. In summary, we demonstrated that glioma cells differentially express functional inwardly rectifying K+ channels suggesting that impaired K+ buffering in cells lacking functional Ba2+-sensitive K+ currents may be a risk factor for increased excitability and thereby contribute to the differential epileptogenicity of gliomas.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Role of Astrocytic Inwardly Rectifying Potassium (Kir) 4.1 Channels in Epileptogenesis

Astrocytes regulate potassium and glutamate homeostasis via inwardly rectifying potassium (Kir) 4.1 channels in synapses, maintaining normal neural excitability. Numerous studies have shown that dysfunction of astrocytic Kir4.1 channels is involved in epileptogenesis in humans and animal models of epilepsy. Specifically, Kir4.1 channel inhibition by KCNJ10 gene mutation or expressional down-regulation increases the extracellular levels of potassium ions and glutamate in synapses and causes hyperexcitation of neurons. Moreover, recent investigations demonstrated that inhibition of Kir4.1 channels facilitates the expression of brain-derived neurotrophic factor (BDNF), an important modulator of epileptogenesis, in astrocytes. In this review, we summarize the current understanding on the role of astrocytic Kir4.1 channels in epileptogenesis, with a focus on functional and expressional changes in Kir4.1 channels and their regulation of BDNF secretion. We also discuss the potential of Kir4.1 channels as a therapeutic target for the prevention of epilepsy.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

Intellectual Disability and Potassium Channelopathies: A Systematic Review

Intellectual disability (ID) manifests prior to adulthood as severe limitations to intellectual function and adaptive behavior. The role of potassium channelopathies in ID is poorly understood. Therefore, we aimed to evaluate the relationship between ID and potassium channelopathies. We hypothesized that potassium channelopathies are strongly associated with ID initiation, and that both gain- and loss-of-function mutations lead to ID. This systematic review explores the burden of potassium channelopathies, possible mechanisms, advancements using animal models, therapies, and existing gaps. The literature search encompassed both PubMed and Embase up to October 2019. A total of 75 articles describing 338 cases were included in this review. Nineteen channelopathies were identified, affecting the following genes: KCNMA1, KCNN3, KCNT1, KCNT2, KCNJ10, KCNJ6, KCNJ11, KCNA2, KCNA4, KCND3, KCNH1, KCNQ2, KCNAB1, KCNQ3, KCNQ5, KCNC1, KCNB1, KCNC3, and KCTD3. Twelve of these genes presented both gain- and loss-of-function properties, three displayed gain-of-function only, three exhibited loss-of-function only, and one had unknown function. How gain- and loss-of-function mutations can both lead to ID remains largely unknown. We identified only a few animal studies that focused on the mechanisms of ID in relation to potassium channelopathies and some of the few available therapeutic options (channel openers or blockers) appear to offer limited efficacy. In conclusion, potassium channelopathies contribute to the initiation of ID in several instances and this review provides a comprehensive overview of which molecular players are involved in some of the most prominent disease phenotypes.

Downregulation of Astrocytic Kir4.1 Potassium Channels Is Associated with Hippocampal Neuronal Hyperexcitability in Type 2 Diabetic Mice

Epilepsy, characterized by recurrent seizures, affects 1% of the general population. Interestingly, 25% of diabetics develop seizures with a yet unknown mechanism. Hyperglycemia downregulates inwardly rectifying potassium channel 4.1 (Kir4.1) in cultured astrocytes. Therefore, the present study aims to determine if downregulation of functional astrocytic Kir4.1 channels occurs in brains of type 2 diabetic mice and could influence hippocampal neuronal hyperexcitability. Using whole-cell patch clamp recording in hippocampal brain slices from male mice, we determined the electrophysiological properties of stratum radiatum astrocytes and CA1 pyramidal neurons. In diabetic mice, astrocytic Kir4.1 channels were functionally downregulated as evidenced by multiple characteristics including depolarized membrane potential, reduced barium-sensitive Kir currents and impaired potassium uptake capabilities of hippocampal astrocytes. Furthermore, CA1 pyramidal neurons from diabetic mice displayed increased spontaneous activity: action potential frequency was ≈9 times higher in diabetic compared with non-diabetic mice and small EPSC event frequency was significantly higher in CA1 pyramidal cells of diabetics compared to non-diabetics. These differences were apparent in control conditions and largely pronounced in response to the pro-convulsant 4-aminopyridine. Our data suggest that astrocytic dysfunction due to downregulation of Kir4.1 channels may increase seizure susceptibility by impairing astrocytic ability to maintain proper extracellular homeostasis.

Microphysiological systems for recapitulating physiology and function of blood-brain barrier

Central nervous system (CNS) diseases are emerging as a major issue in an aging society. Although extensive research has focused on the development of CNS drugs, the limited transport of therapeutic agents across the blood-brain barrier (BBB) remains a major challenge. Conventional two-dimensional culture dishes do not recapitulate in vivo physiology and real-time observations of molecular transport are not possible in animal models. Recent advances in engineering techniques have enabled the generation of more physiologically relevant in vitro BBB models, and their applications have expanded from fundamental biological research to practical applications in the pharmaceutical industry. In this article, we provide an overview of recent advances in the development of in vitro BBB models, with a particular focus on the recapitulation of BBB function. The development of biomimetic BBB models is postulated to revolutionize not only fundamental biological studies but also drug screening.

Defective bicarbonate reabsorption in Kir4.2 potassium channel deficient mice impairs acid-base balance and ammonia excretion

The kidneys excrete the daily acid load mainly by generating and excreting ammonia but the underlying molecular mechanisms are not fully understood. Here we evaluated the role of the inwardly rectifying potassium channel subunit Kir4.2 (Kcnj15 gene product) in this process. In mice, Kir4.2 was present exclusively at the basolateral membrane of proximal tubular cells and disruption of Kcnj15 caused a hyperchloremic metabolic acidosis associated with a reduced threshold for bicarbonate in the absence of a generalized proximal tubule dysfunction. Urinary ammonium excretion rates in Kcnj15- deleted mice were inappropriate to acidosis under basal and acid-loading conditions, and not related to a failure to acidify urine or a reduced expression of ammonia transporters in the collecting duct. In contrast, the expression of key proteins involved in ammonia metabolism and secretion by proximal cells, namely the glutamine transporter SNAT3, the phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase enzymes, and the sodium-proton exchanger NHE-3 was inappropriate in Kcnj15-deleted mice. Additionally, Kcnj15 deletion depolarized the proximal cell membrane by decreasing the barium-sensitive component of the potassium conductance and caused an intracellular alkalinization. Thus, the Kir4.2 potassium channel subunit is a newly recognized regulator of proximal ammonia metabolism. The kidney consequences of its loss of function in mice support the proposal for KCNJ15 as a molecular basis for human isolated proximal renal tubular acidosis.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Kir4.1/Kir5.1 in the DCT plays a role in the regulation of renal K+ excretion

The aim of this mini review is to provide an overview regarding the role of inwardly rectifying potassium channel 4.1 (Kir4.1)/Kir5.1 in regulating renal K+ excretion. Deletion of Kir4.1 in the kidney inhibited thiazide-sensitive NaCl cotransporter (NCC) activity in the distal convoluted tubule (DCT) and slightly suppressed Na-K-2Cl cotransporter (NKCC2) function in the thick ascending limb (TAL). Moreover, increased dietary K+ intake inhibited, whereas decreased dietary K+ intake stimulated, the basolateral potassium channel (a Kir4.1/Kir5.1 heterotetramer) in the DCT. The alteration of basolateral potassium conductance is essential for the effect of dietary K+ intake on NCC because deletion of Kir4.1 in the DCT abolished the effect of dietary K+ intake on NCC. Since potassium intake-mediated regulation of NCC plays a key role in regulating renal K+ excretion and potassium homeostasis, the deletion of Kir4.1 caused severe hypokalemia and metabolic alkalosis under control conditions and even during increased dietary K+ intake. Finally, recent studies have suggested that the angiotensin II type 2 receptor (AT2R) and bradykinin-B2 receptor (BK2R) are involved in mediating the effect of high dietary K+ intake on Kir4.1/Kir5.1 in the DCT.

Downregulation of M-channels in lateral habenula mediates hyperalgesia during alcohol withdrawal in rats

Hyperalgesia often occurs in alcoholics, especially during abstinence, yet the underlying mechanisms remain elusive. The lateral habenula (LHb) has been implicated in the pathophysiology of pain and alcohol use disorders. Suppression of m-type potassium channels (M-channels) has been found to contribute to the hyperactivity of LHb neurons of rats withdrawn from chronic alcohol administration. Here, we provided evidence that LHb M-channels may contribute to hyperalgesia. Compared to alcohol naïve counterparts, in male Long-Evans rats at 24-hours withdrawal from alcohol administration under the intermittent access paradigm for eight weeks, hyperalgesia was evident (as measured by paw withdrawal latencies in the Hargreaves Test), which was accompanied with higher basal activities of LHb neurons in brain slices, and lower M-channel protein expression. Inhibition of LHb neurons by chemogenetics, or pharmacological activation of M-channels, as well as overexpression of M-channels' subunit KCNQ3, relieved hyperalgesia and decreased relapse-like alcohol consumption. In contrast, chemogenetic activation of LHb neurons induced hyperalgesia in alcohol-naive rats. These data reveal a central role for the LHb in hyperalgesia during alcohol withdrawal, which may be due in part to the suppression of M-channels and, thus, highlights M-channels in the LHb as a potential therapeutic target for hyperalgesia in alcoholics.

Targeted deletion of β1-syntrophin causes a loss of Kir 4.1 from Müller cell endfeet in mouse retina

Proper function of the retina depends heavily on a specialized form of retinal glia called Müller cells. These cells carry out important homeostatic functions that are contingent on their polarized nature. Specifically, the Müller cell endfeet that contact retinal microvessels and the corpus vitreum show a tenfold higher concentration of the inwardly rectifying potassium channel K_ir 4.1 than other Müller cell plasma membrane domains. This highly selective enrichment of K_ir 4.1 allows K+ to be siphoned through endfoot membranes in a special form of spatial buffering. Here, we show that K_ir 4.1 is enriched in endfoot membranes through an interaction with β1-syntrophin. Targeted disruption of this syntrophin caused a loss of K_ir 4.1 from Müller cell endfeet without affecting the total level of K_ir 4.1 expression in the retina. Targeted disruption of α1-syntrophin had no effect on K_ir 4.1 localization. Our findings show that the K_ir 4.1 aggregation that forms the basis for K+ siphoning depends on a specific syntrophin isoform that colocalizes with K_ir 4.1 in Müller endfoot membranes.

Inwardly Rectifying Potassium Channel Kir4.1 as a Novel Modulator of BDNF Expression in Astrocytes

Brain-derived neurotrophic factor (BDNF) is a key molecule essential for neural plasticity and development, and is implicated in the pathophysiology of various central nervous system (CNS) disorders. It is now documented that BDNF is synthesized not only in neurons, but also in astrocytes which actively regulate neuronal activities by forming tripartite synapses. Inwardly rectifying potassium (Kir) channel subunit Kir4.1, which is specifically expressed in astrocytes, constructs Kir4.1 and Kir4.1/5.1 channels, and mediates the spatial potassium (K⁺) buffering action of astrocytes. Recent evidence illustrates that Kir4.1 channels play important roles in bringing about the actions of antidepressant drugs and modulating BDNF expression in astrocytes. Although the precise mechanisms remain to be clarified, it seems likely that inhibition (down-regulation or blockade) of astrocytic Kir4.1 channels attenuates K⁺ buffering, increases neuronal excitability by elevating extracellular K⁺ and glutamate, and facilitates BDNF expression. Conversely, activation (up-regulation or opening) of Kir4.1 channels reduces neuronal excitability by lowering extracellular K⁺ and glutamate, and attenuates BDNF expression. Particularly, the former pathophysiological alterations seem to be important in epileptogenesis and pain sensitization, and the latter in the pathogenesis of depressive disorders. In this article, we review the functions of Kir4.1 channels, with a focus on their regulation of spatial K⁺ buffering and BDNF expression in astrocytes, and discuss the role of the astrocytic Kir4.1-BDNF system in modulating CNS disorders.

G protein-coupled receptors differentially regulate glycosylation and activity of the inwardly rectifying potassium channel Kir7.1

Kir7.1 is an inwardly rectifying potassium channel with important roles in the regulation of the membrane potential in retinal pigment epithelium, uterine smooth muscle, and hypothalamic neurons. Regulation of G protein-coupled inwardly rectifying potassium (GIRK) channels by G protein-coupled receptors (GPCRs) via the G protein βγ subunits has been well characterized. However, how Kir channels are regulated is incompletely understood. We report here that Kir7.1 is also regulated by GPCRs, but through a different mechanism. Using Western blotting analysis, we observed that multiple GPCRs tested caused a striking reduction in the complex glycosylation of Kir7.1. Further, GPCR-mediated reduction of Kir7.1 glycosylation in HEK293T cells did not alter its expression at the cell surface but decreased channel activity. Of note, mutagenesis of the sole Kir7.1 glycosylation site reduced conductance and open probability, as indicated by single-channel recording. Additionally, we report that the L241P mutation of Kir7.1 associated with Lebers congenital amaurosis (LCA), an inherited retinal degenerative disease, has significantly reduced complex glycosylation. Collectively, these results suggest that Kir7.1 channel glycosylation is essential for function, and this activity within cells is suppressed by most GPCRs. The melanocortin-4 receptor (MC4R), a GPCR previously reported to induce ligand-regulated activity of this channel, is the only GPCR tested that does not have this effect on Kir7.1.

Oligodendrocyte-encoded Kir4.1 function is required for axonal integrity

Glial support is critical for normal axon function and can become dysregulated in white matter (WM) disease. In humans, loss-of-function mutations of KCNJ10, which encodes the inward-rectifying potassium channel KIR4.1, causes seizures and progressive neurological decline. We investigated Kir4.1 functions in oligodendrocytes (OLs) during development, adulthood and after WM injury. We observed that Kir4.1 channels localized to perinodal areas and the inner myelin tongue, suggesting roles in juxta-axonal K⁺ removal. Conditional knockout (cKO) of OL-Kcnj10 resulted in late onset mitochondrial damage and axonal degeneration. This was accompanied by neuronal loss and neuro-axonal dysfunction in adult OL-Kcnj10 cKO mice as shown by delayed visual evoked potentials, inner retinal thinning and progressive motor deficits. Axon pathologies in OL-Kcnj10 cKO were exacerbated after WM injury in the spinal cord. Our findings point towards a critical role of OL-Kir4.1 for long-term maintenance of axonal function and integrity during adulthood and after WM injury.

Expression of Kir2.1 Inward Rectifying Potassium Channels in Optic Nerve Glia: Evidence for Heteromeric Association with Kir4.1 and Kir5.1

Inward rectifying potassium (Kir) channels comprise a large family with diverse biophysical properties. A predominant feature of central nervous system (CNS) glia is their expression of Kir4.1, which as homomers are weakly rectifying channels, but form strongly rectifying channels as heteromers with Kir2.1. However, the extent of Kir2.1 expression and their association with Kir4.1 in glia throughout the CNS is unclear. We have examined this in astrocytes and oligodendrocytes of the mouse optic nerve, a typical CNS white matter tract. Western blot and immunocytochemistry demonstrates that optic nerve astrocytes and oligodendrocytes express Kir2.1 and that it co-localises with Kir4.1. Co-immunoprecipitation analysis provided further evidence that Kir2.1 associate with Kir4.1 and, moreover, Kir2.1 expression was significantly reduced in optic nerves and brains from Kir4.1 knock-out mice. In addition, optic nerve glia express Kir5.1, which may associate with Kir2.1 to form silent channels. Immunocytochemical and co-immunoprecipitation analyses indicate that Kir2.1 associate with Kir5.1 in optic nerve glia, but not in the brain. The results provide evidence that astrocytes and oligodendrocytes may express heteromeric Kir2.1/Kir4.1 and Kir2.1/Kir5.1 channels, together with homomeric Kir2.1 and Kir4.1 channels. In astrocytes, expression of multiple Kir channels is the biophysical substrate for the uptake and redistribution of K+ released during neuronal electrical activity known as ‘potassium spatial buffering’. Our findings suggest a similar potential role for the diverse Kir channels expressed by oligodendrocytes, which by way of their myelin sheaths are intimately associated with the sites of action potential propagation and axonal K+ release.

Biochemical and Functional Interplay Between Ion Channels and the Components of the Dystrophin-Associated Glycoprotein Complex

Dystrophin is a cytoskeleton-linked membrane protein that binds to a larger multiprotein assembly called the dystrophin-associated glycoprotein complex (DGC). The deficiency of dystrophin or the components of the DGC results in the loss of connection between the cytoskeleton and the extracellular matrix with significant pathophysiological implications in skeletal and cardiac muscle as well as in the nervous system. Although the DGC plays an important role in maintaining membrane stability, it can also be considered as a versatile and flexible molecular complex that contribute to the cellular organization and dynamics of a variety of proteins at specific locations in the plasma membrane. This review deals with the role of the DGC in transmembrane signaling by forming supramolecular assemblies for regulating ion channel localization and activity. These interactions are relevant for cell homeostasis, and its alterations may play a significant role in the etiology and pathogenesis of various disorders affecting muscle and nerve function.

Down-regulation of Inwardly Rectifying K+ Currents in Astrocytes Derived from Patients with Monge's Disease

Chronic mountain sickness (CMS) or Monge's disease is a disease in highlanders. These patients have a variety of neurologic symptoms such as migraine, mental fatigue, confusion, dizziness, loss of appetite, memory loss and neuronal degeneration. The cellular and molecular mechanisms underlying CMS neuropathology is not understood. In the previous study, we demonstrated that neurons derived from CMS patients' fibroblasts have a decreased expression and altered gating properties of voltage-gated sodium channel. In this study, we further characterize the electrophysiological properties of iPSC-derived astrocytes from CMS patients. We found that the current densities of the inwardly rectifying potassium (Kir) channels in CMS astrocytes (-5.7 ± 2.2 pA/pF at -140 mV) were significantly decreased as compared to non-CMS (-28.4 ± 3.4 pA/pF at -140 mV) and sea level subjects (-28.3 ± 5.3 pA/pF at -140 mV). We further demonstrated that the reduced Kir current densities in CMS astrocytes were caused by their decreased protein expression of Kir4.1 and Kir2.3 channels, while single channel properties (i.e., P_o, conductance) of Kir channel in CMS astrocytes were not altered. In addition, we found no significant differences of outward potassium currents between CMS and non-CMS astrocytes. As compared to non-CMS and sea level subjects, the K⁺ uptake ability in CMS astrocytes was significantly decreased. Taken together, our results suggest that down-regulation of Kir channels and the resulting decreased K⁺ uptake ability in astrocytes could be one of the major molecular mechanisms underlying the neurologic manifestations in CMS patients.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Inwardly Rectifying K+ Currents in Cultured Oligodendrocytes from Rat Optic Nerve are Insensitive to pH

Inwardly rectifying K⁺ (Kir) channel expression signals at an advanced stage of maturation during oligodendroglial differentiation. Knocking down their expression halts the generation of myelin and produces severe abnormalities in the central nervous system. Kir4.1 is the main subunit involved in the tetrameric structure of Kir channels in glial cells; however, the precise composition of Kir channels expressed in oligodendrocytes (OLs) remains partially unknown, as participation of other subunits has been proposed. Kir channels are sensitive to H⁺; thus, intracellular acidification produces Kir current inhibition. Since Kir subunits have differential sensitivity to H⁺, we studied the effect of intracellular acidification on Kir currents expressed in cultured OLs derived from optic nerves of 12-day-old rats. Unexpectedly, Kir currents in OLs (2-4 DIV) did not change within the pH range of 8.0-5.0, as observed when using standard whole-cell voltage-clamp recording or when preserving cytoplasmic components with the perforated patch-clamp technique. In contrast, low pH inhibited astrocyte Kir currents, which was consistent with the involvement of the Kir4.1 subunit. The H⁺-insensitivity expressed in OL Kir channels was not intrinsic because Kir cloning showed no difference in the sequence reported for the Kir4.1, Kir2.1, or Kir5.1 subunits. Moreover, when Kir channels were heterologously expressed in Xenopus oocytes they behaved as expected in their general properties and sensitivity to H⁺. It is therefore concluded that Kir channel H⁺-sensitivity in OLs is modulated through an extrinsic mechanism, probably by association with a modulatory component or by posttranslational modifications.

Detection of potassium channel KIR4.1 antibodies in Multiple Sclerosis patients

The presence of KIR4.1 antibodies has been proposed to be a characteristic of Multiple Sclerosis (MS). This could have a significant impact on disease management. However, the validation of the initial findings has failed till date. Conflicting results have been attributed to difficulties in isolating the lower-glycosylated (LG) KIR4.1 expressed in oligodendrocytes, the putative target antigen of autoantibodies. The aim of this study is to verify the presence of KIR4.1 antibodies in MS patients, by independently replicating the originally-described procedure. Assay procedure consisted of KIR4.1 expression in HEK293 cells, 3-step elution to isolate LG-KIR4.1 in elution fraction 3, and ELISA. Sera of 48 MS patients and 46 HCs were studied in 21 working sessions. In a preliminary analysis, we observed different KIR4.1 antibody levels between MS patients and Healthy Controls (HCs). However, a high variability across working sessions was observed and the sensitivity of the assay was very low. Thus, stringent criteria were established in order to identify working sessions in which the pure LG-KIR4.1 was isolated. As per these criteria, we detected LG-KIR4.1 antibodies in 28% of MS patients and 5% of HCs. Unlike previous findings, this study is in agreement with the original report. We propose further efforts be made towards the development of a uniform method to establish the detection of KIR4.1 antibodies in MS patients.

Astrocytic modulation of neuronal excitability through K+ spatial buffering

The human brain contains two major cell populations, neurons and glia. While neurons are electrically excitable and capable of discharging short voltage pulses known as action potentials, glial cells are not. However, astrocytes, the prevailing subtype of glia in the cortex, are highly connected and can modulate the excitability of neurons by changing the concentration of potassium ions in the extracellular environment, a process called K⁺ clearance. During the past decade, astrocytes have been the focus of much research, mainly due to their close association with synapses and their modulatory impact on neuronal activity. It has been shown that astrocytes play an essential role in normal brain function including: nitrosative regulation of synaptic release in the neocortex, synaptogenesis, synaptic transmission and plasticity. Here, we discuss the role of astrocytes in network modulation through their K⁺ clearance capabilities, a theory that was first raised 50 years ago by Orkand and Kuffler. We will discuss the functional alterations in astrocytic activity that leads to aberrant modulation of network oscillations and synchronous activity.

Expression of Kir4.1 and Kir5.1 inwardly rectifying potassium channels in oligodendrocytes, the myelinating cells of the CNS

The inwardly rectifying K+ channel subtype Kir5.1 is only functional as a heteromeric channel with Kir4.1. In the CNS, Kir4.1 is localised to astrocytes and is the molecular basis of their strongly negative membrane potential. Oligodendrocytes are the specialised myelinating glia of the CNS and their resting membrane potential provides the driving force for ion and water transport that is essential for myelination. However, little is known about the ion channel profile of mature myelinating oligodendrocytes. Here, we identify for the first time colocalization of Kir5.1 with Kir4.1 in oligodendrocytes in white matter. Immunolocalization with membrane-bound Na+/K+-ATPase and western blot of the plasma membrane fraction of the optic nerve, a typical CNS white matter tract containing axons and the oligodendrocytes that myelinate them, demonstrates that Kir4.1 and Kir5.1 are colocalized on oligodendrocyte cell membranes. Co-immunoprecipitation provides evidence that oligodendrocytes and astrocytes express a combination of homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. Genetic knock-out and shRNA to ablate Kir4.1 indicates plasmalemmal expression of Kir5.1 in glia is largely dependent on Kir4.1 and the plasmalemmal anchoring protein PSD-95. The results demonstrate that, in addition to astrocytes, oligodendrocytes express both homomeric Kir4.1 and heteromeric Kir4.1/Kir5.1 channels. In astrocytes, these channels are essential to their key functions of K+ uptake and CO2/H+ chemosensation. We propose Kir4.1/Kir5.1 channels have equivalent functions in oligodendrocytes, maintaining myelin integrity in the face of large ionic shifts associated with action potential propagation along myelinated axons.

Glia: A Neglected Player in Non-invasive Direct Current Brain Stimulation

Non-invasive electrical brain stimulation by application of direct current (DCS) promotes plasticity in neuronal networks in vitro and in in vivo. This effect has been mainly attributed to the direct modulation of neurons. Glia represents approximately 50% of cells in the brain. Glial cells are electrically active and participate in synaptic plasticity. Despite of that, effects of DCS on glial structures and on interaction with neurons are only sparsely investigated. In this perspectives article we review the current literature, present own dose response data and provide a framework for future research from two points of view: first, the direct effects of DCS on glia and second, the contribution of glia to DCS related neuronal plasticity.

The role of glial-specific Kir4.1 in normal and pathological states of the CNS

Kir4.1 is an inwardly rectifying K(+) channel expressed exclusively in glial cells in the central nervous system. In glia, Kir4.1 is implicated in several functions including extracellular K(+) homeostasis, maintenance of astrocyte resting membrane potential, cell volume regulation, and facilitation of glutamate uptake. Knockout of Kir4.1 in rodent models leads to severe neurological deficits, including ataxia, seizures, sensorineural deafness, and early postnatal death. Accumulating evidence indicates that Kir4.1 plays an integral role in the central nervous system, prompting many laboratories to study the potential role that Kir4.1 plays in human disease. In this article, we review the growing evidence implicating Kir4.1 in a wide array of neurological disease. Recent literature suggests Kir4.1 dysfunction facilitates neuronal hyperexcitability and may contribute to epilepsy. Genetic screens demonstrate that mutations of KCNJ10, the gene encoding Kir4.1, causes SeSAME/EAST syndrome, which is characterized by early onset seizures, compromised verbal and motor skills, profound cognitive deficits, and salt-wasting. KCNJ10 has also been linked to developmental disorders including autism. Cerebral trauma, ischemia, and inflammation are all associated with decreased astrocytic Kir4.1 current amplitude and astrocytic dysfunction. Additionally, neurodegenerative diseases such as Alzheimer disease and amyotrophic lateral sclerosis demonstrate loss of Kir4.1. This is particularly exciting in the context of Huntington disease, another neurodegenerative disorder in which restoration of Kir4.1 ameliorated motor deficits, decreased medium spiny neuron hyperexcitability, and extended survival in mouse models. Understanding the expression and regulation of Kir4.1 will be critical in determining if this channel can be exploited for therapeutic benefit.